Bone plate with captive clips

ABSTRACT

A fixation system includes a plate comprising a top surface, a bottom surface, a central longitudinal axis, at least one fixation hole extending between the top and bottom surfaces and comprising an undercut therein, and at least one passage intersecting one of the undercuts. At least one resilient clip is disposed in at least a portion of the undercut, with the at least one clip having a pair of generally parallel sides and an end tab. At least one fastener is provided and comprises a head and a threaded shaft, with the head comprising a perimetral groove extending around at least a portion thereof and an instrument receiving portion. The at least one clip is configured and dimensioned to seat in the undercut with the end tab extending within the passage, and the at least one fastener is configured and dimensioned to be received in the at least one fixation hole and securable therein when the at least one clip abuts the perimetral groove of the head.

CROSS-REFERENCE TO RELATION APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/653,164, filed Sep. 2, 2003, the entirety of which isincorporated by reference herein.

FIELD OF THE INVENTION

The invention is related to a fixation system. More particularly, theinvention is related to a plate with a clip for resisting post-operativefastener back-out.

BACKGROUND OF THE INVENTION

Orthopaedic fixation devices such as plates are frequently coupled tobone with fasteners inserted through plate holes. It is known thatsecuring such fasteners to the bone plate, for example through the useof expansion-head screws, can decrease the incidence of loosening of thefixation assembly post-operatively. It is also known that a bushing maybe disposed in each plate hole to receive the fastener to permitpolyaxial movement so that the fastener may be angulated at asurgeon-selected angle. However, typically as the fastener is insertedinto bone through the bushing and plate hole, a threaded head of thefastener engages a threaded internal surface of the bushing to expandthe bushing against the wall of the plate hole, thereby locking thescrew at a given angular orientation with respect to the plate.

Despite these fixation systems, there exists a need for bone plates thatallow post-operative angulation and/or movement. In particular, withrespect to fixation of the spine, there exists a need for anteriorcervical compression plates and associated fasteners that allow thevertebral bodies to compress over grafts post-operatively. In order forcompression to occur, there exists a need for a plate/fastener constructthat allows for translational and/or rotational settling that both occurpost-operatively between the plate and fasteners that have been rigidlyplaced into vertebral bodies. More particularly, there exists a need fora fixation system that facilitates translational settling by permittinga fastener to slide within a plate hole. There further exists a need fora fixation system that facilitates rotational settling by permitting afastener head to toggle or pivot within the plate hole. In addition,there exists a need for a fixation system that permits fastener motionassociated with translational and/or rotational settling while alsoresisting back-out of the fastener from the plate.

SUMMARY OF THE INVENTION

The invention relates to a fixation system that includes a platecomprising a top surface, a bottom surface, a central longitudinal axis,at least one fixation hole extending between the top and bottom surfacesand comprising an undercut therein, and at least one passageintersecting one of the undercuts. At least one resilient clip isdisposed in at least a portion of the undercut, with the at least oneclip having a pair of generally parallel sides and an end tab. At leastone fastener is provided and comprises a head and a threaded shaft, withthe head comprising a perimetral groove extending around at least aportion thereof and an instrument receiving portion. The at least oneclip is configured and dimensioned to seat in the undercut with the endtab extending within the passage, and the at least one fastener isconfigured and dimensioned to be received in the at least one fixationhole and securable therein when the at least one clip abuts theperimetral groove of the head.

The fixation system may further comprise a slot extending through thecentral longitudinal axis. The slot may be aligned along the centrallongitudinal axis or may be disposed transverse to the centrallongitudinal axis. The slot may include a dog-bone shape or the slot mayinclude a pair of overlapping circular shapes. Also, the at least onepassage may extend transverse to at least one of the fixation holes.

The instrument receiving portion of the head of the at least onefastener may at least partially intersect the undercut. A portion of theundercut may be disposed closer to the top surface than the bottomsurface, and may extend completely around the at least one fixationhole.

Each of the fixation holes may further include a lower portion disposedbetween the bottom surface and the undercut, with the lower portiontapering toward a central axis of the fixation hole. The taper of thelower portion may be semispherical. The lower portion also may have afirst maximum inner dimension and the undercut may have a second maximuminner dimension, wherein the first maximum inner dimension is less thanthe second maximum inner dimension.

The head of the fastener may further include a tapered portion disposedbetween the threaded shaft and the perimetral groove, with the taperedportion tapering toward a central axis of the fastener.

At least one fixation hole may be circular. Also, the plate may includeat least two pairs of fixation holes. One pair of fixation holes may begenerally circular and the other pair of fixation holes may be generallyoblong.

In addition, the perimetral groove of the fastener may be interrupted byat least one corner, at least two corners, or at least four corners.Each corner may be configured and dimensioned as a cam to abut an innerwall of the clip. The perimetral groove of the fastener may include anupper surface and a lower surface disposed at between about 10° andabout 70° with respect to each other. In some embodiments, theperimetral groove includes an upper surface and a lower surface disposedat between about 30° and about 50° with respect to each other, and insome embodiments, the perimetral groove includes an upper surface and alower surface aligned at about 40° with respect to each other.

The instrument receiving portion of the head of the at least onefastener may intersect the perimetal groove in at least one location, atleast two locations, or at least four locations. The instrumentreceiving portion of the head of the at least one fastener may have twosubstantially perpendicular slots. Also, the instrument receivingportion may include an internal thread that may extend within the shaft.

The clip may be generally wishbone-shaped. In some embodiments, the clipincludes a generally circular portion or a generally arcuate portion.The clip may also include a discontinuity. In some embodiments the clipmay be configured and dimensioned to slide in the undercut, while inother embodiments the clip may be configured and dimensioned to be fixedand stationary in the plate.

The undercut of the plate may be sized to retain the clip at leastpartially therein while permitting expansion thereof. The at least onefixation hole may include at least two pairs of fixation holes, with atleast one of the pairs of fixation holes being configured anddimensioned to permit toggling of fasteners disposed therein. The headof the fastener may be configured and dimensioned to permit toggling inthe at least one fixation hole, or the head may be configured anddimensioned for coupling to the plate at a fixed angle.

The fastener may be permitted to toggle. The head of the fastener mayfurther include at least one scallop disposed proximate a top peripheralportion of the head. In some embodiments, four scallops are provided.The at least one scallop may include an arcuate portion.

At least one surface of the head may include roughening for interactingwith the clip. In some embodiments, the roughening is formed by steps ona surface.

The at least one resilient clip may be configured and dimensioned topermit toggling of the at least one fastener through a greater angularrange in a cephalad-caudal direction of the plate than in otherdirections of the plate. In particular, the at least one resilient clipmay be configured and dimensioned to permit toggling through a greaterangular range in a cephalad-caudal direction of the plate than in amedial-lateral direction of the plate. Toggling of the at least onefastener may be permitted between about 0° and about 32° along a planeextending parallel to the central longitudinal axis of the plate, whiletoggling of the at least one fastener may be permitted between about 0°and about 20° along a plane extending perpendicular to the centrallongitudinal axis of the plate.

The invention also relates to a fixation system including a plate havinga top surface, a bottom surface, a central longitudinal axis, and a slotextending through the central longitudinal axis. The plate may also haveat least two pairs of fixation holes, with each of the fixation holesextending between the top and bottom surfaces and including an undercuttherein. In addition, at least one passage may extend transverse to oneof the fixation holes and intersect one of the undercuts. The fixationsystem may also include a resilient clip disposed in at least one of theundercuts, with the clip having a pair of generally parallel sides andan end tab. At least one fastener having a head and a threaded shaft maybe provided, with the head including a perimetral groove extendingaround at least a portion thereof and an instrument receiving portionthat at least partially intersects the groove. Each clip may beconfigured and dimensioned to seat in an undercut with the end tabextending within the passage, and each fastener may be configured anddimensioned to be received in one of the fixation holes and securabletherein when the clip abuts the perimetral groove of the head.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred features of the present invention are disclosed in theaccompanying drawings, wherein:

FIG. 1A shows a top view of a plate for use with a first embodiment of afixation system;

FIG. 1B shows a partial cross-sectional view taken along line IB-IB ofthe plate of FIG. 1A;

FIG. 1C shows a partial cross-sectional view taken along line IC-IC ofthe plate of FIG. 1B;

FIG. 1D shows a partial cross-sectional view taken along line ID-ID ofthe plate of FIG. 1C;

FIG. 1E shows another partial cross-sectional view taken along lineID-ID of the plate of FIG. 1C;

FIG. 2A shows a captive clip for use with the plate of FIG. 1A;

FIG. 2B shows a cross-sectional view taken along line IIB-IIB of thecaptive clip of FIG. 2A;

FIG. 3 shows a partial cross-sectional perspective view of the plate ofFIG. 1A with fasteners installed therewith;

FIG. 4A shows a perspective view of an embodiment of a fastener;

FIG. 4B shows a side view of the fastener of FIG. 4A;

FIG. 4C shows a partial cross-sectional side view taken along lineIVC-IVC of the fastener of FIG. 4B;

FIG. 4D shows a partial side view of the head of the fastener of FIG.4A;

FIG. 4E shows a top view of the fastener of FIG. 4A;

FIG. 4F shows a bottom view of the fastener of FIG. 4A;

FIG. 4G shows a partial cross-sectional side view of the shaft andthreads of the fastener of FIG. 4A;

FIG. 5A shows a top view of a plate for use with a different embodimentof a fixation system;

FIG. 5B shows a partial cross-sectional view taken along line VB-VB ofthe plate of FIG. 5A;

FIG. 5C shows a partial cross-sectional view taken along line VC-VC ofthe plate of FIG. 5B;

FIG. 5D shows a partial cross-sectional view taken along line VD-VD ofthe plate of FIG. 5C;

FIG. 5E shows another partial cross-sectional view taken along lineVE-VE of the plate of FIG. 5A;

FIG. 6A shows a circular captive clip for use with the plate of FIG. 5A;

FIG. 6B shows a cross-sectional view taken along line VIB-VIB of thecircular captive clip of FIG. 6A;

FIG. 7A shows an oblong captive clip for use with the plate of FIG. 5A;

FIG. 7B shows a cross-sectional view taken along line VIIB-VIIB of thecircular captive clip of FIG. 7A;

FIG. 8A shows a perspective view of an embodiment of a self-drillingfastener;

FIG. 8B shows a side view of the fastener of FIG. 8A;

FIG. 8C shows a partial cross-sectional side view taken along lineVIIIC-VIIIC of the fastener of FIG. 8B;

FIG. 8D shows a partial side view of the head of the fastener of FIG.8A;

FIG. 8E shows a top view of the fastener of FIG. 8A;

FIG. 8F shows a bottom view of the fastener of FIG. 8A;

FIG. 8G shows a partial cross-sectional side view of the shaft andthreads of the fastener of FIG. 8A;

FIG. 9A shows a top view of a plate for use with a third embodiment of afixation system;

FIG. 9B shows a partial cross-sectional view taken along line IXB-IXB ofthe plate of FIG. 9A;

FIG. 9C shows a top view of end 221 a of the plate of FIG. 9A;

FIG. 9D shows a top view of end 221 b of the plate of FIG. 9A;

FIG. 9E shows a partial cross-sectional view taken along line IXE-IXE ofthe plate of FIG. 9B;

FIG. 9F shows a partial cross-sectional view taken along line IXF-IXF ofthe plate of FIG. 9E;

FIG. 9G shows another partial cross-sectional view taken along lineIXF-IXF of the plate of FIG. 9E;

FIG. 10A shows a captive clip for use with the plate of FIG. 9A;

FIG. 10B shows a cross-sectional view taken along line XB-XB of thecaptive clip of FIG. 10A;

FIG. 11 shows a perspective view of another plate with fastenersinstalled therewith along with the captive clip of FIG. 10A;

FIG. 12 shows an alternate captive clip for use with the plate of FIG.9A or 11;

FIG. 13 shows a partial cross-sectional side view of the captive clip ofFIG. 12 installed in the plate of FIG. 11;

FIG. 14 shows another alternate captive clip;

FIG. 15 shows yet another alternate captive clip;

FIG. 16A shows a bottom perspective view of a plate and cam element usedin a fourth embodiment of a fixation system;

FIG. 16B shows a partial cross-sectional perspective view of the plateand cam element of FIG. 16A with the cam element in a locked positionwith respect to a captive clip;

FIG. 16C shows a bottom view of the plate and cam element of FIG. 16Awith the cam element in an unlocked position with respect to a captiveclip;

FIG. 16D shows a bottom view of the plate and cam element of FIG. 16Awith the cam element in a locked position with respect to a captiveclip;

FIG. 17A shows a top view of a plate for use with a fifth embodiment ofa fixation system;

FIG. 17B shows a partial cross-sectional view taken along lineXVIIB-XVIIB of the plate of FIG. 17A;

FIG. 17C shows a partial cross-sectional view taken along lineXVIIC-XVIIC of the plate of FIG. 17B;

FIG. 17D shows a partial cross-sectional view taken along lineXVIID-XVIID of the plate of FIG. 17C;

FIG. 17E shows another partial cross-sectional view taken along lineXVIID-XVIID of the plate of FIG. 17C;

FIG. 18A shows a captive clip for use with the plate of FIG. 17A;

FIG. 18B shows a cross-sectional view taken along line XVIIIB-XVIIIB ofthe captive clip of FIG. 18A;

FIG. 19A shows a perspective view of another embodiment of a fastener;

FIG. 19B shows a side view of the fastener of FIG. 19A;

FIG. 19C shows a partial cross-sectional side view taken along lineXIXC-XIXC of the fastener of FIG. 19B;

FIG. 19D shows a partial side view of the head of the fastener of FIG.19A;

FIG. 19E shows a top view of the fastener of FIG. 19A;

FIG. 19F shows a bottom view of the fastener of FIG. 19A;

FIG. 19G shows a partial cross-sectional side view of the shaft andthreads of the fastener of FIG. 19A;

FIG. 20A shows a schematic of the captive clip of FIG. 18A in anunexpanded state with a cross-section of a screw head inserted throughthe clip;

FIG. 20B shows a schematic of the captive clip of FIG. 18A in anexpanded state with a cross-section of a screw head inserted through theclip;

FIG. 21A shows a top view of a plate for use with a sixth embodiment ofa fixation system;

FIG. 21B shows a partial cross-sectional view taken along line XXIB-XXIBof the plate of FIG. 21A;

FIG. 21C shows a partial cross-sectional view taken along line XXIC-XXICof the plate of FIG. 21B;

FIG. 21D shows a partial cross-sectional view taken along line XXID-XXIDof the plate of FIG. 21C;

FIG. 21E shows another partial cross-sectional view taken along lineXXID-XXID of the plate of FIG. 21C;

FIG. 22 shows a partial cross-sectional perspective view of the plate ofFIG. 21A with a fastener and captive clip installed therein;

FIG. 23A shows a top view of a one level plate for use with a seventhembodiment;

FIG. 23B shows a partial cross-sectional side view taken along lineXXIIIB-XXIIIB of the plate FIG. 23A;

FIG. 24A shows a top view of a two level plate for use with a seventhembodiment;

FIG. 24B shows a partial cross-sectional side view taken along lineXXIVB-XXIVB of the plate FIG. 24A;

FIG. 25A shows a top view of a three level plate for use with a seventhembodiment;

FIG. 25B shows a partial cross-sectional side view taken along lineXXVB-XXVB of the plate FIG. 25A;

FIG. 26A shows a top view of a four level plate for use with a seventhembodiment;

FIG. 26B shows a partial cross-sectional side view taken along lineXXVIB-XXVIB of the plate FIG. 26A;

FIG. 27A shows a cross-sectional view taken along line XXVIIA-XXVIIA ofthe plate of FIG. 23B;

FIG. 27B shows a partial bottom view of a fixation hole and passage ofthe plate of FIG. 23A;

FIG. 27C shows a partial side view of a portion of the slot of the plateas indicated at detail N in FIG. 23B;

FIG. 27D shows a cross-sectional side view taken along lineXXVIID-XXVIID of the plate of FIG. 23B;

FIG. 27E shows another cross-sectional side view of one of the fixationholes taken along line XXVIID-XXVIID of the plate of FIG. 23B;

FIG. 28A shows a captive clip for use with the plates of FIGS. 23A, 24A,25A, and 26A;

FIG. 28B shows a cross-sectional view taken along line XXVIIIB-XXVIIIBof the captive clip of FIG. 28A;

FIG. 29 shows a partial cross-sectional perspective view of the plate ofFIGS. 23A, 24A, 25A, and 26A with a fastener and the captive clip ofFIG. 28A installed therein;

FIG. 30A shows another embodiment of a captive clip;

FIG. 30B shows a cross-sectional view taken along line XXXB-XXXB of thecaptive clip of FIG. 30A;

FIG. 30C shows another view of the captive clip of FIG. 30A along with across-section of a fastener head taken along the minor diameter of aperimetral groove in the fastener head;

FIG. 31A shows a perspective view of a four level plate for use with aneighth embodiment;

FIG. 31B shows a partial perspective view of the plate of FIG. 31A withcaptive clips and a fastener installed therein;

FIG. 31C shows a partial cross-sectional perspective view of the plateof FIG. 31A with a captive clip and a fastener installed therein;

FIG. 31D shows another partial cross-sectional perspective view of theplate of FIG. 31A with captive clips and fasteners installed therein;

FIG. 31E shows a top view of another embodiment of a plate;

FIG. 31F shows a partial cross-sectional side view of the plate of FIG.31E;

FIG. 31G shows a cross-sectional view taken along line XXXIG-XXXIG ofthe plate of FIG. 31E;

FIG. 31H shows a cross-sectional view taken along line XXXIH-XXXIH ofthe plate of FIG. 31E;

FIG. 31I shows a cross-sectional view taken along line XXXII-XXXII ofthe plate of FIG. 31E;

FIG. 31J shows a cross-sectional view of a keyhole-shaped fixation holeof the plate of FIG. 31E taken in the circular portion of the fixationhole along line XXXIG-XXXIG;

FIG. 31K shows a partial cross-sectional side view taken along themidline 1084 of the plate of FIG. 31E proximate a circular fixationhole;

FIG. 31L shows a partial bottom view of a circular fixation hole of theplate of FIG. 31E;

FIG. 31M shows a partial cross-sectional side view of a keyhole-shapedfixation hole of the plate of FIG. 31E taken in the straight portion ofthe fixation hole along line XXXIH-XXXIH;

FIG. 31N shows a top view of another multi-level fixation plate;

FIG. 31O shows a top view of another embodiment of a captive clip;

FIG. 31P shows a cross-sectional side view taken along line XXXP-XXXP ofthe plate of FIG. 31E;

FIG. 32A shows a side view of an embodiment of a variable angle,self-tapping fastener;

FIG. 32B shows a partial cross-sectional side view taken along lineXXXIIB-XXXIIB of the fastener of FIG. 32A;

FIG. 32C shows a partial side view of the head of the fastener of FIG.32A;

FIG. 32D shows a top view of the fastener of FIG. 32A;

FIG. 32E shows a partial cross-sectional side view of the shaft andthreads of the fastener of FIG. 32A configured and dimensioned for usewith cancellous bone;

FIG. 32F shows another partial cross-sectional side view of the shaftand threads of the fastener of FIG. 32A configured and dimensioned foruse with cortical bone;

FIG. 33A shows a side view of an embodiment of a variable angle,self-drilling fastener;

FIG. 33B shows a partial cross-sectional side view taken along lineXXXIIIB-XXXIIIB of the fastener of FIG. 33A;

FIG. 33C shows a partial side view of the head of the fastener of FIG.33A;

FIG. 33D shows a top view of the fastener of FIG. 33A;

FIG. 34A shows a side view of an embodiment of a fixed angle,self-tapping fastener;

FIG. 34B shows a partial cross-sectional side view taken along lineXXXIVB-XXXIVB of the fastener of FIG. 34A;

FIG. 34C shows a partial side view of a fixed-angle head of the fastenerof FIG. 34A;

FIG. 34D shows a top view of the fastener of FIG. 34A;

FIG. 34E shows a partial cross-sectional side view taken along lineXXXIVF-XXXIVF of the fastener of FIG. 34A;

FIG. 35A shows a side view of an embodiment of a fixed angle,self-drilling fastener;

FIG. 35B shows a partial cross-sectional side view taken along lineXXXVB-XXXVB of the fastener of FIG. 35A;

FIG. 35C shows a partial side view of the head of the fastener of FIG.35A;

FIG. 35D shows a top view of the fastener of FIG. 35A;

FIG. 35E shows a partial cross-sectional side view taken along lineXXXVF-XXXVF of the fastener of FIG. 35A;

FIG. 36A shows a first perspective view of an embodiment of a platehaving a fastener with scallops installed therewith;

FIG. 36B shows another perspective view of the embodiment of FIG. 36A;

FIG. 36C shows another perspective view of the embodiment of FIG. 36A;

FIG. 36D shows another perspective view of the embodiment of FIG. 36A;

FIG. 37A shows a side view of an embodiment of a fastener;

FIG. 37B shows a partial cross-sectional side view taken along lineXXXVIIB-XXXVIIB of the fastener of FIG. 37A;

FIG. 37C shows a partial side view of the head of the fastener of FIG.37A;

FIG. 37D shows a partial cross-sectional side view taken along lineXXXVIID-XXXVIID of the fastener of FIG. 37E;

FIG. 37E shows a top view of the fastener of FIG. 37A;

FIG. 37F shows a partial top view of the fastener of FIG. 37A

FIG. 37G shows a partial side view of area S of FIG. 37C;

FIG. 37H shows an alternate a top view of the fastener of FIG. 37A;

FIG. 37I shows an alternate partial top view of the fastener of FIG.37A;

FIG. 38A shows a perspective view of an embodiment of a plate havinggussets;

FIG. 38B shows a top view of the plate of FIG. 38A;

FIG. 38C shows a cross-sectional side view taken along lineXXXVIIIC-XXXVIIIC of the plate of FIG. 38A;

FIG. 38D shows a cross-sectional side view taken along lineXXXVIIID-XXXVIIID of the plate of FIG. 38A, with a fastener disposed ina hole in the plate;

FIG. 38E shows a perspective view of an embodiment of a fastener withslits;

FIG. 38F shows a cross-sectional side view of the fastener of FIG. 38Ewith a fastener head in an expanded state;

FIG. 38G shows a cross-sectional side view of the fastener of FIG. 38Ewith a fastener head in a contracted state as being inserted or removedfrom the plate;

FIG. 39A shows a perspective view of another embodiment of a fixationsystem;

FIG. 39B shows a top view of the plate of the fixation system shown inFIG. 39A;

FIG. 39C shows a side view of the plate of FIG. 39B;

FIG. 39D shows a cross-sectional view of the plate of FIG. 39C takenalong the line X39-X39;

FIG. 40 shows a top view of an embodiment of a clip;

FIG. 41A shows a side view of an exemplary variable-angle screw;

FIG. 41B shows a side view of an exemplary fixed-angle screw;

FIG. 42A shows a partially-removed cross-sectional view of avariable-angle screw engaging a plate;

FIG. 42B shows a partially-removed cross-sectional view of a fixed-anglescrew engaging a plate;

FIG. 43A shows a side view of an exemplary variable-angle drill guidefor use with the system of FIG. 39A;

FIG. 43B shows a cross-sectional view of the barrel of the drill guideof FIG. 43A;

FIG. 43C shows an enlarged partial cross-sectional view of the tipportion of the drill guide of FIG. 43A;

FIG. 44A shows a side view of an exemplary fixed-angle drill guide foruse with the system of FIG. 39A;

FIG. 44B shows a cross-sectional view of the barrel of the drill guideof FIG. 44A;

FIG. 44C shows an enlarged partial cross-sectional view of the tipportion of the drill guide of FIG. 44A; and

FIG. 44D shows a partially-removed cross-sectional view of a fixed-angledrill guide engaging a plate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning to FIGS. 1A-4G, a first embodiment of a fixation system isshown. The fixation system includes a plate 20 with two pairs offixation holes 22, 24. Fixation holes 22 are oblong in shape so as tohave a different length than width (i.e., the hole is non-circular),while fixation holes 24 are circular in shape. The fixation holes arepreferably oblong in the direction along the longitudinal axis 28 a ofthe bone plate 20. Although plate 20 is provided with two pairs offixation holes 22, 24, more than two pairs may instead be provided, forexample so that plate 20 may span a greater length and thus be fastenedto multiple locations along the spine. Single holes alternatively may beprovided as opposed to pairs.

A slot 26 is aligned along central longitudinal axis 28 a for receivinga drill/screw guide and for graft visualization. Preferably, slot 26does not receive any fasteners. In alternate embodiments, more than oneslot may be provided, and the slot or slots may be disposed transverseto central longitudinal axis 28 a. Preferably, slot 26 includes straightportions 26 a and semicircular portions 26 b.

Each of fixation holes 22, 24 extends between top and bottom surfaces28, 30 and includes an undercut 32. In one embodiment, undercut 32 isdisposed closer to top surface 28 than bottom surface 30 while inanother embodiment, undercut 32 is disposed closer to bottom surface 30than top surface 28. Undercut 32 also may be disposed intermediate topand bottom surfaces 28, 30, or transverse to the surfaces. Preferably,an undercut 32 extends completely around each of fixation holes 22, 24.A lower portion 34 of each fixation hole 22, 24 may be disposed betweenbottom surface 28 and undercut 32 and may taper toward a central axis 36of the respective fixation hole as shown for example in FIG. 1D.Preferably, lower portion 34 has a first maximum inner dimension L1 andthe undercut has a second maximum inner dimension 12, wherein the firstmaximum inner dimension L1 is less than the second maximum innerdimension L2.

Turning to FIGS. 2A and 2B, a generally circular captive clip 50 isshown with a slit 52 that permits elastic expansion/compression of clip50 as will be explained shortly, as well as an outer edge 54 and aninner edge 56. Clip 50 includes a generally rectangular cross-section,and in one embodiment, is provided with a radius R1 adjoining outer edge54 of about 0.1 mm and a second radius R2 adjoining inner edge 56 ofabout 0.2 mm.

Referring to FIG. 3, plate 20 is shown with a fastener 60 in each offixation holes 22, 24. A captive clip 50 is provided for each fastener60 around head 62. Preferably, a clip 50 is pre-installed in eachfixation hole 22, 24, and snap-fits around a head 62 of a fastener 60such that post-operative back-out of a fastener 60 from a hole 22, 24 isresisted. Preferably, a captive clip 50 disposed in an undercut 32 of anelongated, oblong hole 22 is permitted to travel (slide) along alongitudinal axis 22 a of the hole, as shown in FIGS. 1A and 1E. Thus, afastener 60 on a captive clip 50 and disposed in an oblong hole 22 canslide across the length of hole 22. However, a captive clip 50 disposedin an undercut 32 of a circular hole 24 is not permitted to travel(slide) along a longitudinal axis 24 a of hole 24, a shown in FIGS. 1Aand 1D.

As will be described in detail herein, various embodiments of fastenersare contemplated. In particular, two main types of fasteners for“variable angle” and “fixed angle” applications are provided.

“Variable angle” refers to fasteners and/or plates for which: (1) thetrajectory of insertion of the fastener into bone (through a fixationhole in the plate) may be selected by the surgeon (although only alimited range of motion may be permitted); and/or (2) the trajectory ofthe fastener with respect to the plate is allowed to change followinginsertion into bone, for example to toggle to accommodate anytranslational and/or rotational settling that occur post-operativelybetween the plate and the fastener that has been rigidly placed into avertebral body (although only a limited range of motion may bepermitted).

“Fixed angle” refers to fasteners and/or plates for which: (1) thetrajectory of insertion of the fastener into bone (through a fixationhole in the plate) is pre-selected and thus fixed; and/or (2) thetrajectory of the fastener with respect to the plate is not allowed tochange following insertion into bone.

Each of the two main types of fasteners may be provided with featuresfor use preferably with either cancellous or cortical bone. Moreover,each of the two main types of fasteners may be provided preferably witheither self-tapping or self-drilling features. Finally, the diameters ofthe fasteners may be varied as well as the lengths. In selecting afastener for a given application, therefore, a surgeon must decide whichtype of fastener and which combination of features are appropriate. Athe minimum, the surgeon must select among eight categories offasteners: (1) variable-angle, cancellous, self-tapping, (2)variable-angle, cancellous, self-drilling, (3) variable-angle, cortical,self-tapping, (4) variable-angle, cortical, self-drilling, (5)fixed-angle, cancellous, self-tapping, (6) fixed-angle, cancellous,self-drilling, (7) fixed-angle, cortical, self-tapping, and (8)fixed-angle, cortical, self-drilling.

As shown in FIGS. 4A-4G, an exemplar fastener 60 includes a head 62 witha self-tapping, threaded shaft 64 with a flute 65 provided proximate thebottom of the threading. In an alternate embodiment, as described above,a self-drilling, threaded shaft 64 instead may be provided. Head 62includes a perimetral groove 66 extending around at least a portionthereof, and an instrument receiving portion 68 that at least partiallyintersects groove 66 at one or more openings 70. Instrument receivingportion 68 preferably includes an internal, unthreaded, cylindrical,annular wall 69, although in an alternate embodiment wall 69 may bethreaded. In the exemplar embodiment of FIG. 4E, instrument receivingportion 68 is cross-shaped and thus intersects groove 66 at fouropenings 70. A pair of slots 70 a, 70 b (similar to the look of aPhillips-head screw) form instrument receiving portion 68. However, inalternate embodiments, only one opening 70, two openings 70, or anynumber of openings 70 may be provided. Fastener 60 preferably is usedwith a plate 20 such that fastener 62 may toggle in the fixation holes.In addition, fastener 60 preferably is used for fixation to cancellousbone. However, threading appropriate for cortical bone instead may beprovided.

In one preferred exemplary embodiment, perimetral groove 66 includes anupper surface 66 a and a lower surface 66 b disposed at an angle δ ofbetween about 10° and about 70° with respect to each other. Morepreferably, upper surface 66 a and lower surface 66 b are disposed at anangle δ of between about 30° and about 50° with respect to each other,and most preferably, they are an angle δ of about 40° with respect toeach other. The angulation of surfaces 66 a, 66 b with respect to eachother permits toggling of fastener 60 when coupled to a captive clip 50.

Head 62 preferably is partially spherical and includes a bottom section68 extending to the top end 70 of shaft 64, with bottom section 68tapering inward toward longitudinal axis 72 from perimetral groove 66 totop end 70. Advantageously, such tapering permits angulation of fastener60 when disposed in a fixation hole 22, 24.

In use, a fastener 60 is received in a captive clip 50. The shaft 64 isinitially screwed into bone until the partial-spherical head 62 offastener 60 reaches captive clip 50. Upon further insertion of fastener60 into captive clip 50, the partial-spherical head 62, particularlybottom section 68, bears against the inside edge 56 of captive clip 50and expands captive clip 50. Once fastener 60 is inserted far enough,captive clip 50 contracts so that it “snaps” into perimetral groove 66in head 62, thereby preventing fastener 60 from backing out of plate 20,as previously described.

In addition, captive clip 50 subsequently may be elastically expanded topermit removal of fastener 60. When a screwdriver tip is inserted in theslots 70 a, 70 b of instrument receiving portion 68, the screwdriver tipprotrudes into the perimetral groove 66 through openings 70. Insertingthe screwdriver tip thus elastically expands captive clip 50 to allowfastener 60 to be unscrewed from bone without interference from captiveclip 50.

A second embodiment of a fixation system is shown in FIGS. 5A-8G. Thefixation system includes a plate 120 with two pairs of fixation holes122, 124. Fixation holes 122 are oblong in shape so as to have adifferent length than width (i.e., the hole is non-circular), whilefixation holes 124 are circular in shape. Although plate 120 is providedwith two pairs of fixation holes 122, 124, more than two pairs mayinstead be provided, for example so that plate 120 may span a greaterlength and thus be fastened to multiple locations along the spine.

Plate 120 is not provided with a slot for receiving a drill/screw guideor for graft visualization. However, in alternate embodiments, one ormore slots may be provided, and the slot or slots may be disposedtransverse to central longitudinal axis 128 a.

Each of fixation holes 122, 124 extends between top and bottom surfaces128, 130 and includes an undercut 132. In one embodiment, undercut 132is disposed closer to top surface 128 than bottom surface 130 while inanother embodiment, undercut 132 is disposed closer to bottom surface130 than top surface 128. Undercut 132 also may be disposed intermediatetop and bottom surfaces 128, 130, or transverse to the surfaces.Preferably, an undercut 132 extends completely around each of fixationholes 122, 124. A lower portion 134 of each fixation hole 122, 124 maybe disposed between bottom surface 128 and undercut 132. Lower portion134 optionally may taper toward a central axis 136 of the respectivefixation hole as shown for example in FIG. 5D. Preferably, lower portion134 has a first maximum inner dimension L3 and the undercut has a secondmaximum inner dimension L4, wherein the first maximum inner dimension L3is less than the second maximum inner dimension L4.

Plate 120 may be provided with two types of captive clips. First, acircular captive clip 150 a is shown in FIGS. 6A and 6B with a slit 152a that permits elastic expansion/compression of clip 150 a as previouslyexplained, as well as an outer edge 154 a and an inner edge 156 a. Clip150 a includes a generally rectangular cross-section, and in oneembodiment, is provided with a radius R3 on outer edge 154 a of about0.1 mm and a second radius R4 on inner edge 156 a of about 0.2 mm. In arelaxed state, ends 158 a, 159 a forming slit 152 a of clip 150 apreferably are angled at between about 5° and about 15° with respect toeach other, and more preferably at about 10°. Clip 150 a is sized to bereceived in fixation hole 124. Second, an oblong captive clip 150 b isshown in FIGS. 7A and 7B with a slit 152 b that permits elasticexpansion/compression of clip 150 b as previously explained, as well asan outer edge 154 b and an inner edge 156 b. Clip 150 b includes agenerally rectangular cross-section, and in one embodiment, is providedwith a radius R5 at outer edge 154 b of about 0.1 mm and a second radiusR6 at inner edge 156 b of about 0.2 mm. Clip 150 b is sized to bereceived in a fixation hole 122. In a relaxed state, ends 158 b, 159 bforming slit 152 b of clip 150 b preferably are angled at between about5° and about 15° with respect to each other, and more preferably atabout 10°.

In an exemplary preferred embodiment, a captive clip 150 a or 150 b isprovided for each fastener 160 around head 162, based on whether thefastener 160 is disposed in an oblong fixation hole 122 or a circularfixation hole 124. Preferably, a clip 150 is pre-installed in eachfixation hole 122, 124, and snap-fits around at least part of a head 162of a fastener 160 during installation of the fastener such thatpost-operative back-out of a fastener 160 from a hole 122, 124 isresisted. Preferably, when a captive clip 150 b is disposed in undercut132 of elongated, oblong hole 122, captive clip 150 b serves as a railupon which a fastener 160 is permitted to travel (slide) along alongitudinal axis 122 b of the hole. Thus, a fastener 160 on a captiveclip 150 b and disposed in an oblong hole 122 can slide across thelength of the hole. However, neither captive clip 150 a disposed in anundercut 132 of a circular hole 124, nor a fastener captured by thecaptive clip 150 a, is permitted to travel (slide) along a longitudinalaxis 124 a of the hole.

Turning to FIGS. 8A-8G, an exemplar fastener 160 according to thepresent invention includes a head 162 with a self-drilling, threadedshaft 164. In an alternate embodiment, a self-tapping, threaded shaft164 instead may be provided. Head 152 includes a perimetral groove 166extending around at least a portion thereof, and an instrument receivingportion 168. Although instrument receiving portion 168 is not shown toat least partially intersect groove 166 at one or more openings,provision for such may be made as described previously. In the exemplarembodiment of FIG. 8E, instrument receiving portion 168 ishexagonal-shaped. However, in alternate embodiments, other shapes may beprovided. Fastener 160 preferably is used with a plate 120 such thatfastener 160 may toggle in the fixation holes. In addition, fastener 160preferably is used for fixation to cancellous bone.

In one preferred exemplary embodiment, perimetral groove 166 includes anupper surface 166 a and a lower surface 166 b that are disposed atbetween about 5° and about 50° with respect to each other. Preferably,upper surface 166 a is disposed at an angle λ₁ of between about 5° andabout 15° with respect to a line 173 disposed approximately intermediateupper inside edge 173 and lower inside edge 173 b and perpendicular tolongitudinal axis 172. Most preferably, angle λ₁ is about 10°. Inaddition, lower surface 166 b preferably is disposed at an angle λ₂ ofbetween about 15° and about 35° with respect to line 173. Mostpreferably, angle λ₂ is about 25°. Thus, most preferably, angles λ₁, λ₂total about 35°. The angulation of surfaces 166 a, 166 b with respect toeach other permits toggling of fastener 160 when coupled to a captiveclip 150 a, 150 b.

Head 162 preferably is partially spherical and includes a bottom section168 extending to the top 170 of shaft 164, with bottom section 168tapering inward toward longitudinal axis 172 from perimetral groove 166to top 170. Advantageously, such tapering permits angulation of fastener160 when disposed in a fixation hole 122, 124.

In use, a fastener 160 is received in a captive clip 150 a or 150 b. Theshaft 164 is initially screwed into bone until the partial-sphericalhead 162 of fastener 160 reaches captive clip 150 a or 150 b. Uponfurther insertion of fastener 160 into captive clip 150, thepartial-spherical head 162, particularly bottom section 168, bearsagainst the inside edge 156 a, 156 b of captive clip 150 a, 150 b,respectively, and expands the clip. Once fastener 160 is inserted farenough, captive clip 150 a or 150 b contracts so that it “snaps” intoperimetral groove 166 in head 162, thereby preventing fastener 160 frombacking out of plate 120, as previously described.

A third embodiment of a fixation system is shown in FIGS. 9A-12. Thefixation system includes a plate 220 with two pairs of fixation holes222, 224. Fixation holes 222 are oblong in shape, while fixation holes224 are circular in shape. Although plate 220 is provided with two pairsof fixation holes 222, 224, more than two pairs may instead be provided,for example so that plate 220 may span a greater length and thus befastened to multiple locations along the spine.

A slot 226 is aligned along central longitudinal axis 228 a forreceiving a drill/screw guide and for graft visualization. Preferably,slot 226 does not receive any fasteners. In alternate embodiments, morethan one slot may be provided, and the slot or slots may be disposedtransverse to central longitudinal axis 228 a. Preferably, slot 226includes straight portions 226 a and semicircular portions 226 b.

Each of fixation holes 222, 224 extends between top and bottom surfaces228, 230. As shown in particular in FIGS. 9C and 9D, each pair of thefixation holes is partially intersected by a common slot 232.

Turning to FIGS. 10A and 10B, an unslitted captive clip 250 is shown.Preferably, captive clip 250 is elastically flexible to permit expansionand contraction, although such flexibility is not necessary. Clip 250has an outer edge 254 and an inner edge 256. Clip 250 also has agenerally rectangular cross-section, and in one embodiment, has flatupper and lower surfaces 258 a, 258 b, as well as rounded sides 259 a,259 b.

In another embodiment of a plate 220 b, shown in FIG. 11, two pairs ofcircular fixation holes 224 are provided. As explained with respect toplate 220, each pair of fixation holes 224 is partially intersected by acommon slot 232. A captive clip 250 is inserted into each slot 232, andis sized such that it protrudes into each fixation hole 224. Preferably,a clip 250 is pre-installed in each slot 232. Once a fastener, such asone of the fasteners previously described with a perimetral groove inits head, is inserted far enough in the fixation hole, captive clip 250bears against the perimetral groove, thereby locking the fastener inplace and preventing the fastener from backing out of plate 220 b. Inorder to permit removal of a fastener, a captive clip 250 may be removedfrom either the cephalad or caudal side of plate 220 b where thefastener is located, thereby unlocking that fastener from plate 220 b.Plate 220 b is similar to plate 220 as shown in FIGS. 9A-9G, with a clip250 assembled therewith.

An alternate embodiment of a clip 270 is shown in FIGS. 12-13. Clip 270is generally shaped like a paper clip, and thus may confer spring-likebehavior. As with the embodiment of plate 220 b, clip 270 may beinserted in, and/or removed from, a slot 232 from either the cephalad orcaudal side of plate 220 b. Preferably, a first end 272 of clip 270 isarcuate, while a second end 274 is straight. The corners 276 a, 276 b ofend 274 may be rounded, beveled, or provide a sharp transition from sideedges 278 a, 278 b such as a right-angle transition between side edges278 a, 278 b and end 274. Second end 274 is straight for stability, andpreferably is disposed toward the outer edge of the plate. In someembodiments, one or more edge surfaces may be straight to allow for aflush resting surface with a greater surface area for abutting a surfaceof a plate in which clip 270 is installed. Two free ends 279 a, 279 bmay be provided.

Additional alternate embodiments of captive clips are shown in FIGS. 14and 15. A clip 280, shown in FIG. 14, includes a slit 282 that permitselastic expansion/compression of clip 280 as previously explained. Clip280 has an outer edge 284 and an inner edge 286. Clip 290, as shown inFIG. 15, includes a slit 292 that permits elastic expansion/compression,an outer edge 294, an inner edge 296, and further includes opposingrecesses 298 the potential use of which will be explained below. Captiveclip 290 further includes a pair of generally parallel sides 299 a, 299b.

Turning to FIGS. 16A-16D, a fourth embodiment of a fixation system isshown. The fixation system includes a plate 320 with two pairs ofcircular fixation holes 222 (only one pair is shown in FIG. 16A). A slot326 is aligned along central longitudinal axis 328 a for receiving adrill/screw guide and for graft visualization. Preferably, slot 326 doesnot receive any fasteners. Each of fixation holes 322 extends betweentop and bottom surfaces 328, 330. Each pair of fixation holes 322 ispartially intersected by a common slot 332. As shown in each of FIGS.16A-16D, a captive clip 290 is inserted in slot 332 from a side,preferably cephalad or caudal, of plate 320. In an alternate embodiment,slot 332 is disposed transverse to central longitudinal axis 328 a andsuch that it opens on a side of plate 320, preferably the cephaladand/or caudal sides. In addition, slot 332 alternatively may open on aside of plate 320 that extends between the cephalad and caudal sides ofplate 320.

A cam element 334 is inserted from the bottom 330 of plate 320, and isreceived in recesses 298 of captive clip 290 to govern expansion andcontraction thereof. Cam element 334 is turned using a locking screw 336fastened thereto. In its unlocked position, shown in FIGS. 16A and 16C,captive clip 290 can freely expand and contract, and thus a fastener isnot captured as securely by the clip. In its locked position, shown inFIGS. 16B and 16D, cam element 334 engages recesses 298, and thuscompression of captive clip 290 is prevented by cam element 334 therebypermitting captive clip 290 to securely capture one or more fastenersinserted in fixation holes 322. In an exemplary preferred embodiment,cam element 334 has four rounded corners to facilitate engagement withrecesses 298.

Turning to FIGS. 17A-20B, a fifth embodiment of a fixation system isshown. The fixation system includes a plate 420 with two pairs offixation holes 422, 424. Fixation holes 422, 424 are circular in shape.Although plate 420 is provided with two pairs of fixation holes 422,424, more than two pairs may instead be provided, for example so thatplate 420 may span a greater length and thus be fastened to multiplelocations along the spine. A “dogbone” shaped slot 426 is aligned alongcentral longitudinal axis 427 for receiving a drill/screw guide and forgraft visualization. Preferably, slot 426 does not receive anyfasteners.

Each of fixation holes 422, 424 extends between top and bottom surfaces428, 430 and includes an undercut 432. In one embodiment, undercut 432is disposed transverse to top and bottom surfaces 428, 430,respectively, along line 432 a, but other orientations are also possibleas previously described with respect to other embodiments. Preferably,an undercut 432 extends completely around each of fixation holes 422,424. At least one passage 429 extends transverse to each of the fixationholes 422, 424, preferably from bottom surface 428, and intersects anundercut 432 associated with the hole 422 or 424. Passages 429 are usedfor alignment of captive clips, as will be described shortly.

Turning to FIGS. 18A and 18B, a generally U-shaped captive clip 450 isshown with a slit 452 that permits elastic expansion/compression of clip450, as well as an outer edge 454, an inner edge 456, and generallyparallel sides 459 a, 459 b. Clip 450 includes a generally rectangularcross-section, and in one embodiment, is provided with a radius R7 atedges 454 and 456 of between about 0.1 mm and about 0.2 mm. An end tab458 extends from side 459 a, preferably transverse to the plane of thepage. In some embodiments, end tab 458 extends generally parallel to theplane of the page. When a captive clip 450 is installed in a fixationhole 422, 424, end tab 458 extends into a passage 429 so that captiveclip 450 may be aligned and properly placed in undercut 432 in thefixation hole. In an exemplary preferred embodiment, captive clips 450are pre-installed in fixation holes 422, 424 in plate 420 prior toinstallation of fasteners therein.

A captive clip 450 is provided for each fastener 460 around head 462.Preferably, clip 450 snap-fits around head 462 of a fastener 460 suchthat post-operative back-out of a fastener 460 from a hole 422, 424 isresisted. A fastener 460 retained on a captive clip 450 disposed in anundercut 432 of a circular hole 422, 424 may be permitted to togglealong a central axis of the hole.

As shown in FIGS. 19A-19G, an exemplar fastener 460 includes a head 462with a self-tapping, threaded shaft 464. In an alternate embodiment, aself-drilling, threaded shaft 464 may be provided. Head 462 includes aperimetral groove 466 that is interrupted by four corners 470 separatingback wall 471. In the exemplar embodiment of FIG. 19E, an instrumentreceiving portion 468 is hexagonal-shaped. However, in alternateembodiments, other shapes may be provided. Fastener 460 preferably isused for fixation to cancellous bone, although threading appropriate forfixation to cortical bone instead may be provided. In one preferredexemplary embodiment, perimetral groove 466 includes an upper portion466 a and a lower portion 466 b that are disposed generally parallel toeach other.

Head 462 preferably is partially spherical and includes a bottom section475 extending to the top end 473 of shaft 464, with bottom section 475tapering inward toward longitudinal axis 472 from perimetral groove 466to top end 473. Advantageously, such tapering permits angulation offastener 460 when disposed in a fixation hole 422, 424.

In use, fastener 460 is received in captive clip 450. The shaft 464 isinitially screwed into bone until the partial-spherical head 462 offastener 460 reaches captive clip 450. Upon further insertion offastener 460 into captive clip 450, the partial-spherical head 462,particularly bottom section 475, bears against the inside edge 456 ofcaptive clip 450 and expands captive clip 450. Once fastener 460 isinserted far enough, captive clip 450 contracts so that it “snaps” intoperimetral groove 466 in head 462, thereby preventing fastener 460 frombacking out of plate 420, as previously described.

As evident from FIGS. 20A and 20B, in some orientations, the fourcorners 470 of fastener 460 may be retained on captive clip 450 whilethe clip is in an unexpanded state. In other orientations, two of thecorners act as cams on the inner wall 456 of the clip to elasticallyexpand it such that the fastener is no longer locked by captive clip450. Thus, it is possible to remove a fastener by turning the fastenerto engage the cams with the clip to expand the clip.

A fastener 460, retained on captive clip 450, can toggle on captive clip450 in a fixation hole 422, 424, because of the geometry of theperimetral groove 466 in head 462 of fastener 460 and the geometry ofthe fixation hole 422, 424. During toggling, captive clip 450 remainsessentially fixed in place, while the axis of fastener 460 is allowed toangulate with respect to the central axis of the fixation hole 422, 424.In embodiments in which surfaces 466 a, 466 b of groove 466 aregenerally parallel to each other, toggling may be substantially limited.

A sixth embodiment of a fixation system is shown in FIGS. 21A-22. Thefixation system includes a plate 520 with two pairs of fixation holes522, 524. Fixation holes 522, 524 are circular in shape. Although plate520 is provided with two pairs of fixation holes 522, 524, more than twopairs may instead be provided, for example so that plate 520 may span agreater length and thus be fastened to multiple locations along thespine. Although no slot is provided along central longitudinal axis 528a for receiving a drill/screw guide and for graft visualization, one maybe provided as described previously with respect to other embodiments.

Each of fixation holes 522, 524 extends between top and bottom surfaces528, 530, respectively, and includes an undercut 532. In one embodiment,undercut 532 is disposed transverse to top and bottom surfaces 528, 530,respectively, but other orientations are also possible as previouslydescribed with respect to other embodiments. Preferably, an undercut 532extends completely around each of fixation holes 522, 524.

The geometry of one pair of fixation holes 522, preferably the caudalpair of holes, is sized such that movement of a fastener 560 therein isvery limited. The geometry of the remaining pair of fixation holes 524allows a fastener 560 to toggle between about 5° and about 30°, and morepreferably between about 15° and about 20°, as shown in FIG. 22. Inparticular, a fastener 560, shown in FIG. 22, is retained on a captiveclip 550. The fastener 560 may toggle on captive clip 550 in fixationhole 524 because of the geometry of perimetral groove 566 in the head562 of the fastener and the geometry of fixation hole 524. Inparticular, lower portion 534 a widens proximate bottom surface 530 ofplate 520. The captive clip 550 preferably remains fixed in place, whilethe axis of fastener 560 is allowed to angulate with respect to the axisof fixation hole 524.

With reference to FIGS. 23A-29, a seventh embodiment of a fixationsystem is shown. Plates 620, 720, 820, 920 represent “one level,” “twolevel,” “three level,” and “four level” constructions, respectively.Each “level” is provided by one or more fixation holes, and preferablypairs of adjacent fixation holes, bridging two vertebrae. Thus, a firstlevel plate bridges two vertebrae and includes at least one fixationhole, preferably two adjacent fixation holes for each vertebrae, while asecond level plate bridges three vertebrae and includes at least onefixation hole, preferably two adjacent fixation holes for eachvertebrae. For example, plate 620 has two pairs of fixation holes 622,624, and thus only represents one level; in contrast, plate 720 hasthree pairs of fixation holes, and thus the middle pair of fixationholes is adjacent two other pairs thus forming a two level construction.

As can be seen from FIGS. 23A-29, the construction of plates 620, 720,820, 920 is very similar, and thus although plate 620 will be describedin detail herein, the description of plate 620 applies also to plates720, 820, 920.

Referring to FIGS. 23A and 27A-27E, the fixation system includes a plate620 with two pairs of fixation holes 622, 624. Fixation holes 622, 624are circular in shape. A “figure eight” shaped slot 626 is aligned alongcentral longitudinal axis 628 a for receiving a drill/screw guide andfor graft visualization. Preferably, slot 626 does not receive anyfasteners, and is beveled along upper inner edge 626 a at an angle αbetween about 30° and about 60°, and more preferable about 45°. Itshould be noted that as the length of plates 620, 720, 820, 920increases along the central longitudinal axis, slot 626 becomeselongated and generally “dog-bone” shaped as shown for example in FIGS.25A and 26A.

Each of fixation holes 622, 624 extends between top and bottom surfaces628, 630 and includes an undercut 632. In one embodiment, undercut 632is disposed transverse to top and bottom surfaces 628, 630,respectively, but other orientations are also possible as previouslydescribed with respect to other embodiments. Preferably, an undercut 632extends completely around each of fixation holes 622, 624. At least onepassage 629 extends transverse to each of the fixation holes 622, 624,preferably from bottom surface 628, and intersects undercut 632associated with the hole 622 or 624. In some embodiments, passage 629may extend generally parallel to each of the fixation holes 622, 624,preferably from bottom surface 628, and intersects undercut 632associated with the hole 622 or 624. Passages 629 are used for alignmentof captive clips, as will be described shortly. In the preferredexemplary embodiment, holes 629 are disposed along lines generallyparallel to central longitudinal axis 628 a, for example along line 628b.

As shown in FIGS. 28A and 28B, a generally “wishbone-shaped” captiveclip 650 includes a slit 652 that permits elastic expansion/compressionof clip 650, as well as an outer edge 654, an inner edge 656, andgenerally parallel sides 659 a, 659 b. Clip 650 also includes agenerally rectangular cross-section, as shown in FIG. 28B. In apreferred exemplary embodiment, an end tab 658 extends between, andpreferably halfway between, sides 659 a, 659 b and transverse to theplane of the page. In some embodiments, end tab 658 extends generallyparallel to the plane of the page. When a captive clip 650 is installedin a fixation hole 622, 624, end tab 658 extends into passage 629 sothat captive clip 650 may be aligned and properly placed in the fixationhole. Preferably, captive clips 650 are pre-installed in fixation holes622, 624 in plate 620 prior to installation of fasteners therein.

Referring to FIG. 29, a captive clip 650 is provided for fastener 660around head 662. Preferably, clip 650 snap-fits around head 662 offastener 660 such that post-operative back-out of fastener 660 from hole622, 624 is resisted. Preferably, captive clip 650 is disposed in anundercut 632 of a fixation hole 622, 624 and is permitted to togglealong a central axis of the hole, as shown for example in FIG. 29 asindicated for example by arrows A.

Another preferred embodiment of a “wishbone-shaped” captive clip 680 isshown in FIGS. 30A to 30C. Similar to the captive clip 650 shown inFIGS. 28A and 28B, captive clip 680 includes a slit 682 that permitselastic expansion/compression of clip 680, as well as an outer edge 684,an inner edge 686, and generally parallel sides 689 a, 689 b. Clip 680also includes a generally rectangular cross-section, as shown in FIG.30B. In a preferred exemplary embodiment, an end tab 688 extendsbetween, and preferably halfway between, sides 689 a, 689 b, along clipcentral longitudinal axis 688 a and transverse to the plane of the page.In some embodiments, end tab 688 extends generally parallel to the planeof the page. When captive clip 680 is installed in fixation hole 622,624, end tab 688 extends into a passage 629 so that captive clip 680 maybe aligned and properly placed in the fixation hole. Preferably, captiveclips 680 are pre-installed in fixation holes 622, 624 in plate 620prior to installation of fasteners therein. Preferably, the captiveclips 680 are sized and configured so that once installed in the boneplate they are fixed in position. In the embodiment of the clip shown inFIG. 30A, the corners of the clip are wedged into the undercut so thatmovement of the clip in the undercut is prevented. In addition, once thefastener is inserted in the captive clip, the captive clip is even moresecurely wedged into position to prevent movement in the undercut.

A preferred exemplary embodiment of captive clip 680 includes thefollowing dimensions: L5 of about 5.5 mm, L6 of about 4.4 mm, L7 ofabout 2.5 mm, L8 of about 7.2 mm, L9 of about 3 mm, L10 of about 3.8 mm,and L11 of about 2.85 mm. Captive clip 680 also has a thickness L12 ofbetween about 0.3 mm and about 0.4 mm, more preferably about 0.37 mm.Preferred radii of curvature include: R8 of about 0.25 mm, R9 of about2.8 mm, R10 of about 1 mm, R11 of about 0.3 mm, R12 of about 0.4 mm, andR13 of about 6.8 mm. A preferred angle β between outer edges of straightportions 689 a, 689 b and adjacent outer edges of straight portions 690a, 690 b, respectively, is about 150°. Preferably, captive clip 680 issymmetric about central longitudinal axis 688 a.

Advantageously, as shown for example in FIG. 30C, captive clip 680 issized such that a fastener has a greater freedom to toggle in somedirections as compared to other directions. For example, axis 688 a isoriented parallel to the cephalad-caudal direction of a plate, whilecentral axis 692 may be oriented perpendicular to axis 688 and parallelto the medial-lateral direction of the plate. A cross-section of anexemplary fastener head 694 is also shown, with the cross-section takenthrough the center of a perimetral groove in head 694 as described inthe embodiments of fasteners disclosed herein. An outer edge 696 of theperimetral groove in head 694 is relatively close to edge 686 of captiveclip 680 proximate central axis 692, while outer edge 696 is spaced agreater amount from edge 686 of captive clip 680 proximate axis 688 a.Thus, toggling of the fastener is more limited in the direction of axis692 because fastener head 694 may only be angulated until it contactsedge 686 which serves as a stop. In one preferred exemplary embodiment,a fastener inserted in captive clip 680 is permitted to toggle throughbetween about 0° and about 32° along the plane extending perpendicularto the page through line 688 a, while only being permitted to togglethrough between about 0° and about 20° along the plane extendingperpendicular to the page through line 692. Preferably, a fastener ispermitted a greater freedom to toggle in the cephalad-caudal directionof a plate.

The preferred exemplary embodiment of captive clip 680, as well as eachof the other captive clips described herein, preferably is formed ofElgiloy® (Cobalt-Chromium-Nickel alloy), ASTM F-1058 Grade I, burr freeand electropolished. Preferably, each of the captive clips of thepresent invention may have high strength, ductility and good mechanicalproperties including an ultimate tensile strength between about 250,000psi and about 350,000 psi (about 1,700 MPa and 2,000 MPa, respectively),a hardness (HRC) of between about 45 and about 60, an elastic modulus ofup to about 30,000,000 psi (about 270 GPa), excellent fatigue life, andcorrosion resistance. Alternatively, each of the captive clips describedherein may be formed of another elastic material such as Nitinolsuperelastic alloy memory material per ASTM-2063.

Referring to FIGS. 31A to 31D, an eighth embodiment of a fixation systemis shown. A plate 1020 is provided with one or more pair(s) of fixationholes 1022 that are generally “keyhole-shaped” and oblong, and one ormore pair(s) of fixation holes 1024 that are generally circular. Each ofthe circular and keyhole-shaped holes includes an undercut 1026 thatreceives an “omega-shaped” captive clip 1028. Captive clip 1028 includesa pair of generally parallel sides 1030 a, 1030 b and two end tabs 1032a, 1032 b protruding from each side 1030 a, 1030 b, respectively.

The geometry of fixation holes 1022, 1024 and their associated undercuts1026 is sized such that movement of a captive clip 1028 once seated inan undercut 1026 is prevented, except that the undercuts 1026accommodate elastic expansion of captive clip 1028 as previouslyexplained. One tab 1032 a, 1032 b of captive clip 1028 is received in ahole 1034 that extends from undercut 1026 to a side of plate 1020, whilethe other tab 1032 a, 1032 b is received in a hole 1036 that extendsfrom undercut 1026 toward the midline 1036 of plate 1020. Tabs 1032 a,1032 b are used to align the “omega-shaped” captive clip 1028 in plate1020. Preferably, captive clip 1028 is symmetrical about clip centralaxis 1029. Captive clip 1028 also preferably is installed in plate 1020such that central axis 1029 is parallel midline 1036 of plate 1020 withintermediate portion 1031 of clip 1028 oriented so as to not interferewith movement of a fastener 1040 along the length of fixation hole 1022.

The “keyhole” shaped fixation holes 1022 include a rounded portion 1038in which a head 1040 a of a fastener 1040 is received, and a straightportion 1042 with an integrally formed ridge 1044 that may extend aboutthe length of straight portion 1042 and under which the head 1040 a ispermitted to move. Captive clips 1028 are disposed in each roundedportion.

In use, the shaft 1040 b of fastener 1040 is initially screwed into bonethrough the rounded portion 1038 of fixation hole 1022 until thepartial-spherical head 1040 a of fastener 1040 reaches captive clip1028. Upon further insertion of fastener 1040 into captive clip 1028,the partial-spherical head 1040 a bears against the inside edge 1046 ofcaptive clip 1028 and expands the captive clip. Once fastener 1040 isinserted far enough, captive clip 1028 contracts so that it “snaps” intothe perimetral groove 1050 in the head 1040 a, thereby preventingfastener 1040 from backing out of plate 1020, as previously described.Fastener 1040 is then allowed to travel along the length of fixationhole 1022, for example under integrally formed ridge 1044 of the platewhich provides additional resistance to back-out of fastener 1040.However, in order to allow shaft 1040 b to be removed from the bonewithout interference from ridge 1044 on the straight portion 1042 of the“keyhole-shaped” fixation holes 1022, fastener 1040 must be moved sothat head 1040 a is disposed proximate the “omega-shaped” captive clip1028. As described previously, captive clip 1028 may then be elasticallyexpanded to permit removal of fastener 1040. With respect to the pair ofcircular holes 1024, again, the screw may be removed by expansion of thecaptive clip 1028.

Thus, a fastener 1040 disposed in a “keyhole-shaped” fixation hole 1022is permitted to travel (slide) along the longitudinal axis of the hole1022. The fastener 1040 can slide along the parallel sides 1030 a, 1030b of the “omega-shaped” captive clip 1028 or under the integrally formedridge 1044. A fastener disposed in a circular hole may be permitted totoggle but remains relatively stationary also as previously described.

It should be noted that plate 1020 shown in FIG. 31A is a four levelconstruct, and includes five pairs of holes. Plate 1020 thus includesthree additional pairs of “keyhole-shaped” fixation holes 1060, 1062,1064. In alternate embodiments of plate 1020, more than one pair ofcircular fixation holes may be provided, or as few as one“keyhole-shaped” fixation hole may be provided. While no slots areprovided along midline 1036 of plate 1020, slots may be included asdescribed with respect to other plate embodiments.

Another embodiment of a fixation system similar to that shown in FIGS.31A to 31D is shown in FIGS. 31E to 31P. In the “one-level” embodimentshown, a plate 1066 is provided with a pair of fixation holes 1068 thatare generally “keyhole-shaped” and oblong, and a pair of fixation holes1070 that are generally circular. An embodiment of a plate 1066 b withfour pairs of fixation holes 1068 and one pair of fixation holes 1070also is shown in FIG. 31N. Thus, as described herein with respect toother embodiments, each plate may be provided with one or more pairs ofoblong fixation holes and one or more pairs of circular fixation holes.Each of the circular and keyhole-shaped holes includes an undercut 1072that receives an “omega-shaped” captive clip such as captive clip 1074,shown in FIGS. 31O to P. Captive clip 1074 includes a pair of generallyparallel sides 1076 a, 1076 b and two end tabs 1078 a, 1078 b protrudingfrom each side 1076 a, 1076 b, respectively. Also, captive clip 1074includes extensions 1093 a, 1093 b for additional stability. As can beseen from FIG. 31O, the periphery of captive clip 1074 lies entirelywithin circle 1094 except portions of end tabs 1078 a, 1078 b, and thuscaptive clip 1074 is interchangeably useable with keyhole-shaped andcircular holes 1068, 1070, respectively. Advantageously, captive clip1074 may be used both with plates in which fasteners are not permittedto travel along the length of any of the holes, as well as with platesin which fasteners are allowed to move across the fixation holes.

In addition, as shown for example in FIGS. 31E and 31L, at least onepassage 1085 optionally may be provided, and are used for alignment ofcaptive clips as previously described. Thus, for example,wishbone-shaped captive clip 650 optionally may be used in at least onefixation hole 1070 in plate 1066, while “omega-shaped” captive clip 1074optionally at the same time or instead may be used in at least onefixation hole 1068, 1070 in plate 1066. Plate 1066 thereforeaccommodates use of more than one design of captive clip.

A preferred exemplary embodiment of captive clip 1074 includes thefollowing dimensions: M1 of about 7.5 mm, M2 of about 5.96 mm, M3 ofabout 4.4 mm, M4 of about 5.6 mm, M5 of about 0.5 mm, M6 of about 1.4mm, and M7 of about 5.2 mm. Captive clip 1074 also has a thickness M8 ofbetween about 0.3 mm and about 0.4 mm, more preferably about 0.35 mm.Preferred radii of curvature include: R1 a of about 1 mm, R2 a of about0.1 mm, R3 a of about 0.15 mm, R4 a of about 0.5 mm, R5 a of about 2.7mm, and R6 a of about 3.3 mm.

The geometry of fixation holes 1068, 1070 and their associated undercuts1072 is sized such that movement of a captive clip 1074 once seated inan undercut 1072 is prevented, except that the undercuts 1072accommodate elastic expansion of captive clip 1074 as previouslyexplained. One tab 1078 a, 1078 b of captive clip 1074 is received in ahole 1080 that extends from undercut 1072 to a side of plate 1066, whilethe other tab 1078 a, 1078 b is received in a hole 1082 that extendsfrom undercut 1072 toward the midline 1084 of plate 1066. As shown forexample in FIGS. 31G and 31H, holes 1082 from adjacent undercuts ofadjacent fixation holes preferably communicate with each other and maybe circular in cross-section. Tabs 1078 a, 1078 b are used to align the“omega-shaped” captive clip 1074 in plate 1066. Preferably, captive clip1074 is symmetrical about clip central axis 1086. Captive clip 1074 alsopreferably is installed in plate 1066 such that central axis 1086 isparallel to midline 1084 of plate 1066 with intermediate portion 1087 ofclip 1074 oriented so as to not interfere with movement of a fasteneralong the length of fixation hole 1068.

The “keyhole” shaped fixation holes 1068 include a rounded portion 1088in which a head of a fastener is received such as head 1040 a of afastener 1040. Fixation holes 1068 also include a straight portion 1089with an integrally formed ridge 1090 that may extend about the length ofstraight portion 1089 and under which the head 1040 a is permitted tomove. Captive clips 1074 are disposed in each rounded portion.

Plate 1066 further may be provided with a slot 1091 for visualizationand instrument-receiving holes 1092 for receiving a suitably configureddrill guide. Slots 1091 may be any suitable shape including other shapesdisclosed herein such as dog-bone shapes. In addition, for multi-levelplates, the length and/or spacing of the slots may vary as shown in FIG.31N.

The general operation of captive clip 1074 and plate 1066 is asdescribed above with respect to FIGS. 31A to 31D.

Turning to FIGS. 32A to 32F, an exemplar preferred self-tapping fastener1160 is shown preferably for use in variable angle applications.Fastener 1160 includes a head 1162 with a self-tapping, threaded shaft1164 ending in a blunt tip 1164 a. Head 1162 includes a perimetralgroove 1166 extending around at least a portion thereof, and aninstrument receiving portion 1168 that preferably at least partiallyintersects groove 1166 at one or more openings 1170. In the exemplarembodiment of FIG. 32D, instrument receiving portion 1168 iscross-shaped and thus intersects groove 1166 at four openings 1170. Asshown in the figures, exemplar preferred dimensioning for self-tappingfasteners is listed in Tables I-III below. Because fasteners for usewith cancellous bone differ from fasteners for use with cortical boneparticularly with respect to the threads on shaft 1164, FIGS. 32E and32F are provided for cancellous bone and cortical bone, respectively,along with appropriate dimensions in the tables below. TABLE I Dimension(a - cancellous; 4.0 mm and 4.5 mm b - cortical) Self-Tapping Fastener(°) Y₁ about 100 Y₂ about 20 Y₃ about 20 Y₄ about 25 Y_(5a) about 35Y_(5b) about 20 Y_(6a) about 3 Y_(6b) about 20 Y₇ about 120

TABLE II Dimension (a - cancellous; 4.0 mm and 4.5 mm b - cortical)Self-Tapping Fastener (mm) R14 about 1.4 R15 about 12.75 R16 about 0.3R17 about 0.1 R19a about 0.2 R19b about 0.15 R20a about 0.05 R20b about0.05 R21a about 1 R21b about 0.7

TABLE III Dimension 4.0 mm (a - cancellous; Self-Tapping 4.5 mmSelf-Tapping b - cortical) Fastener (mm) Fastener (mm) L13 about 3.9about 3.9 L14 about 3 about 3 L15 about 4 about 4 L16 about 1.84 about1.84 L17 about 6 about 6 L18 about 2.6 about 2.6 L19 about 4 about 4 L20about 3.1 about 3.71 L22 about 3.67 about 3.67 L23 about 4 about 4.5 L24about 3.3 about 3.3 L25 about 5.5 about 5.5 L26 about 4.2 about 4.2 L27about 4 about 4 L28 about 1.2 about 1.2 L29 about 0.5 about 0.5 L30about 0.7 about 0.7 L31 about 5 about 5 L32 about 0.8 about 0.8 L33aabout 0.5 about 0.75 L33b about 0.5 about 0.75 L34a about 1.75 about1.75 L34b about 1.25 about 1.25

Dimensions particularly important for self-tapping fasteners includeL15, L16, L19, L20, R14, R15, and γ₇.

Threading dimensions particularly important for fasteners used withcancellous bone include L33 a, L34 a, R19 a, R20 a, R21 a, γ _(5a), andγ_(6a). Threading dimensions particularly important for fasteners usedwith cortical bone include L33 b, L34 b, R19 b, R20 b, R21 b, γ _(5b),and γ_(6b).

Turning next to FIGS. 33A to 33D, an exemplar preferred self-drillingfastener 1260 is shown preferably for use in variable angleapplications. Fastener 1260 includes a head 1262 with a self-drilling,threaded shaft 1264 ending in a sharp tip 1264 a. Head 1262 includes aperimetral groove 1266 extending around at least a portion thereof, andan instrument receiving portion 1268 that at least partially intersectsgroove 1266 at one or more openings 1270. In the exemplar embodiment ofFIG. 33D, instrument receiving portion 1268 is cross-shaped and thusintersects groove 1266 at four openings 1270. As shown in the figures,exemplar preferred dimensioning for self-drilling fasteners is listed inTables IV-VI below. As discussed previously, fasteners for use withcancellous bone differ from fasteners for use with cortical boneparticularly with respect to the threads on shaft 1264. Thus, thethreads and dimensions shown in FIGS. 32E and provided for cancellousbone and cortical bone, respectively, also may be used with fastener1260, along with appropriate dimensions in the tables below. TABLE IVDimension 4.0 mm and 4.5 mm (a - cancellous; Self-Drilling Cancellousb - cortical) Fastener (°) Y₈ about 100 Y₉ about 50 Y₁₀ about 90 Y₁₁about 20 Y₁₂ about 20 Y₁₃ about 25 Y₁₆ about 50

TABLE V Dimension 4.0 mm and 4.5 mm (a -3 cancellous; Self-DrillingCancellous b - cortical) Fastener (mm) R22 about 0.3 R23 about 0.1

TABLE VI Dimension 4.0 mm (a - cancellous; Self-Drilling 4.5 mmSelf-Drilling b - cortical) Cancellous Fastener (mm) Cancellous Fastener(mm) L35 about 3.9 about 3.9 L36 about 3 about 3 L37 about 3.5 about 3.5L38 about 6 about 6 L39 about 2.6 about 2.6 L40 about 3.5 about 3.5 L41about 0.7 about 1.2 L42 about 4 about 4.5 L44 about 3.67 about 3.67 L45about 3.3 about 3.3 L46 about 5.5 about 5.5 L47 about 4.2 about 4.2 L48about 4 about 4 L49 about 1.2 about 1.2 L50 about 0.5 about 0.5 L51about 0.7 about 0.7 L52 about 5 about 5 L53 about 0.8 about 0.8

Dimensions particularly important for self-drilling fasteners includeL37, L40, L41, γ₉, γ₁₀, and γ₁₆.

With respect to the threaded portion of shaft 1264, the threadspreferably are provided in accordance with the dimensions describedpreviously with respect to fastener 1160 as shown in FIGS. 32E and 32F.

Turning to FIGS. 34A to 34E, another exemplar preferred self-tappingfastener 1360 is shown preferably for use in fixed-angle applications.Fastener 1360 includes a head 1362 with a self-tapping, threaded shaft1364. Head 1362 includes a perimetral groove 1366 extending around atleast a portion thereof, and an instrument receiving portion 1368 thatpreferably at least partially intersects groove 1366 at one or moreopenings 1370. In the exemplar embodiment of FIG. 34E, instrumentreceiving portion 1368 is cross-shaped and thus intersects groove 1366at four openings 1370. Preferably, fastener 1360 is provided forcancellous bone.

As shown in FIGS. 34A to 34E, in one exemplary preferred embodiment,fastener 1360 is provided with dimensioning as listed in Table VIIbelow: TABLE VII 4.0 mm Fixed Angle, Self-Tapping, Cancellous DimensionFastener L60 about 7.5 mm L61 about 6 mm minimum L62 about 2 mm (M2threading) L63 about 1.6 mm L64 about 2.84 mm L65 about 12 mm to about20 mm L66 about 4 mm L67 about 3.1 mm L68 about 3.3 mm L69 about 2.8 mmL70 about 2.5 mm L71 about 2.7 mm L72 about 0.38 mm L73 about 0.7 mm L74about 4.2 mm L75 about 4.4 mm L76 about 4.68 mm L77 about 5.12 mm L78about 1.15 mm

Fastener 1360 is disposed about central longitudinal axis 1372 which isoriented perpendicular to a plane that includes uppermost surface 1362a. Head 1362 preferably is provided with a first surface 1362 b orientedat an angle σ₄ with respect to uppermost surface 1362 a, as well as asecond surface 1362 c oriented at an angle σ₅ with respect to a plane1374 disposed midway across groove 1366 and perpendicular to axis 1372.Groove 1366 is bounded at its upper and lower ends by second and thirdsurfaces 1362 c and 1362 d, respectively. Preferably, third surface 1362d is oriented at an angle σ₆ with respect to a plane 1374 disposedmidway across groove 1366 and perpendicular to axis 1372. A transitionsurface 1362 e also may be disposed at an angle σ₇ with respect to axis1372. Also, a generally flat surface 1362 e connecting surfaces 1362 band 1362 c preferably is oriented at an angle σ₃ with respect to axis1372. Dimensioning for a preferred exemplary embodiment of FIGS. 34A to34E is listed in Table VIII below: TABLE VIII 4.0 mm Fixed Angle,Self-Tapping, Cancellous Dimension Fastener σ₁ about 100° σ₂ about 120°σ₃ about 12° σ₄ about 20° σ₅ about 5° σ₆ about 20° σ₇ about 45° R28about 12.5 mm SR29 about 1.4 mm

With respect to the threaded portion of shaft 1364, the threadspreferably are provided in accordance with the dimensions describedpreviously with respect to fastener 1160 as shown in FIGS. 32E and 32F.

Dimensions particularly important for “fixed-angle” fasteners are shownin FIG. 34C.

As shown in FIGS. 35A to 35E, another exemplar preferred self-drillingfastener 1460 is shown preferably for use in fixed angle applications.Fastener 1460 includes a head 1462 with a self-drilling, threaded shaft1464. Head 1462 includes a perimetral groove 1466 extending around atleast a portion thereof, and an instrument receiving portion 1468 thatpreferably at least partially intersects groove 1466 at one or moreopenings 1470. In the exemplar embodiment of FIG. 35E, instrumentreceiving portion 1468 is cross-shaped and thus intersects groove 1466at four openings 1470. Preferably, fastener 1460 is provided forcancellous bone.

As shown in FIGS. 35A to 35E, in one exemplary preferred embodiment, afixed angle, self-drilling fastener 1460 is provided with dimensioningas listed in Table IX below: TABLE IX 4.0 mm Fixed Angle, Self-Drilling,Cancellous Dimension Fastener L79 (internal thread) about 7.5 mm L80(internal thread) about 6 minimum L81 about 2 mm (M2 threading) L82about 1.6 mm L83a about 2.84 mm L83b about 12 mm to about 20 mm L84aabout 3.5 mm L84b about 0.7 mm L86 about 3.3 mm L87 about 2.8 mm L88about 2.5 mm L89 about 2.7 mm L90 about 0.38 mm L91 about 0.7 mm L92about 4.4 mm L93 about 4 mm L94 about 4.68 mm L95a about 5.12 mm L95babout 1.15 mm

Fastener 1460 is disposed about central longitudinal axis 1472 which isoriented perpendicular to a plane that includes uppermost surface 1462a. Head 1462 preferably is provided with a first surface 1462 b orientedat an angle σ_(10b) with respect to uppermost surface 1462 a, as well asa second surface 1462 c oriented at an angle σ_(10c) with respect to aplane 1474 disposed midway across groove 1466 and perpendicular to axis1472. Groove 1466 is bounded at its upper and lower ends by second andthird surfaces 1462 c and 1462 d, respectively. Preferably, thirdsurface 1462 d is oriented at an angle σ_(10d) with respect to a plane1474 disposed midway across groove 1466 and perpendicular to axis 1472.Also, a generally flat transition surface 1462 e connecting surfaces1462 b and 1462 c preferably is oriented at an angle σ_(10a) withrespect to axis 1472. Dimensioning for a preferred exemplary embodimentof FIGS. 35A to 35E is listed in Table X below: TABLE X 4.0 mm FixedAngle, Self-Drilling, Cancellous Dimension Fastener σ_(10a) about 12°σ_(10b) about 20° σ_(10c) about 5° σ_(10d) about 20° σ_(10e) about 45°

With respect to the threaded portion of shaft 1464, the threadspreferably are provided in accordance with the dimensions describedpreviously with respect to fastener 1160 as shown in FIGS. 32E and 32F.

Some surfaces of head 1462 of fastener 1460 may be provided withfeatures such as roughening in the form of grooves, rounded bumps,ridges, steps, serrations, etc., to provide tactile and/or audiofeedback when a captive clip interacts therewith. For example, in onepreferred exemplary embodiment, surfaces 1462 c, 1462 d, and 1462 f maybe provided with such roughening.

When comparing the fixed angle fastener 1360 of FIGS. 34A to 34E withthe variable angle fastener 1160 of FIGS. 32A to 32F, preferably thefollowing conditions are met: maximum shaft diameter L76>maximum shaftdiameter L23; maximum head diameter L78 b>maximum head diameter L25; andgroove vertical width L72<groove vertical width L29.

Dimensions particularly important for “fixed angle” fasteners are shownin FIG. 35C.

In yet another exemplary embodiment of an exemplary fastener 1560, shownin FIGS. 36A to 36D, head 1562 is provided with scallops 1563 so that acaptive clip 1580 in a fixation plate 1582 may be visualized by asurgeon, thereby allowing the surgeon to confirm that head 1562 has beencaptured by captive clip 1580 to resist backout from fastener hole 1584.In one embodiment, scallops 1563 are arcuate in shape. However, othershapes may be provided as long as visualization of captive clip 1580 ispermitted when fastener 1560 is coupled thereto. Preferably, fourequally radially spaced scallops are provided proximate a top peripheralportion of head 1562.

Further details concerning exemplary fastener 1560 are shown in FIGS.37A to 37F. Fastener 1560 includes a head 1562 with a self-tapping,threaded shaft 1564. Head 1562 includes a perimetral groove 1566extending around at least a portion thereof, and an instrument receivingportion 1568 that preferably at least partially intersects groove 1566at one or more openings 1570. In the exemplar embodiment of FIG. 37E,instrument receiving portion 1568 is cross-shaped and thus intersectsgroove 1566 at four openings 1570. Fastener 1560 is disposed aboutcentral longitudinal axis 1572 which is oriented perpendicular to aplane that includes uppermost surface 1562 a. Preferably, fastener 1560is provided for cancellous bone.

In an exemplary embodiment, the dimensioning listed in Table IX andTable X for fastener 1460, as well as the threads and dimensioningtherefor as disclosed for example with respect to FIG. 35D, also areapplicable to fastener 1560. As shown in FIG. 37C, however, fastener1560 is further provided with a generally cylindrical portion 1562 gdisposed between surface 1562 d with rounded transition 1562 e andsurface 1562 f. Cylindrical portion 1562 g may be provided with adiameter L96 of about 4.9 mm. In addition, the distance betweenuppermost surface 1562 a and the plane defined at the intersection ofrounded transition 1562 e and surface 1562 g may be provided with alength L97 of about 1.8 mm. Finally, surface 1562 e may transition tosurface 1562 g at an angle σ₁₃ of between about 0° and about 45°, andmore preferably about 20°. Further detail of area S in FIG. 37C is shownin FIG. 37G. Radiuses R37 and R38 of about 0.1 mm may be provided. Thus,in operation, when a fastener abuts a clip while being installed in aplate, the clip crosses from the tapered surface 1562 f to the flatcylindrical surface 1562 g, and then catches on surface 1562 e. Such aconfiguration permits tactile feedback while the fastener is interactingwith the clip.

As shown in FIGS. 37E and 37F, instrument receiving portion 1568preferably has a maximum width L98 of about 5 mm and a width L99 of eachsection 1568 a of about 1 mm. Preferably, the midway points of adjacentscallops 1563 are disposed about 90° from each other. Moreover, themidway point of each scallop 1563 is disposed at an angle σ₁₄ of about45° from a line extending through the midway points of opposingoutermost portions 1570. Tangents from rounded corners 1563 a, 1563 b ofeach scallop 1563 preferably are disposed at about 50° with respect toeach other, with corners 1563 a, 1563 b being provided with a radius R35of about 0.1 mm. Each scallop preferably has a radius R36 of about 0.75mm and extends inward toward central longitudinal axis 1472 so that theradial distance L99 between the innermost portion of scallop 1563 andthe central longitudinal axis 1572 may be about 2.1 mm.

Turning to FIGS. 37H to 37I, a variation of the fastener head shown inFIG. 37E is shown. In particular, scallops 1563B are each provided witha central portion 1580 having a radius R39 that is bordered by adjacentportions 1582 having a radius R40. Also, each of the portions ofcross-shaped instrument receiving portion 1568B may extend to theperiphery of the screw head and thus open along the upper portionthereof as shown at openings 1584.

In an exemplary preferred embodiment, each of the fasteners and fixationplates disclosed herein may be formed of a titanium alloy such astitanium-aluminum-niobium, preferably anodized. One preferred materialfor use with each of the plates and screws described herein isTi-6Al-7Nb, with a density of about 4.52 gm/cc, a modulus of elasticityof about 105 GPa, an ultimate tensile strength of about 900 MPa, and ayield strength of about 800 MPa. Surfaces of the fasteners preferablyalso are burr free, with all sharp edges broken to a maximum of 0.1 mm.

Each of the embodiments of plates, fasteners, and clips disclosed hereinmay be provided in sets or kits. For example, as shown in FIG. 23A, eachof the exemplary “one-level” plates according to the present inventionmay be provided with maximum lengths X₁, lengths X₂ between the centersof cephalad and caudal holes, and lengths X₃ between the center of theplate along longitudinal axis 628 a and the center of a fastener hole,as follows: TABLE XI Exemplary Plate X₁ X₂ X₃ 1 about 22.5 mm about 14.0mm about 7.0 mm 2 about 24.5 mm about 16.0 mm about 8.0 mm 3 about 26.5mm about 18.0 mm about 9.0 mm 4 about 28.5 mm about 20.0 mm about 10.0mm 5 about 30.5 mm about 22.0 mm about 11.0 mm 6 about 32.5 mm about24.0 mm about 12.0 mm 7 about 34.5 mm about 26.0 mm about 13.0 mm

Thus, kits of plates may be provided including two or more plates, suchas plates selected from exemplary plates 1-7 as listed in Table XI. Forexample, plates may be provided with lengths X₁ that differ by about 2mm each, lengths X₂ that differ by about 2 mm each, and lengths X₃ thatdiffer by about 1 mm each.

Similarly, as shown for example in FIG. 24A, each of the exemplary“two-level” plates according to the present invention may be providedwith maximum lengths X₁, lengths X₂ between the centers of cephalad andcaudal holes, and lengths X₃ between the centers of adjacent holes, asfollows: TABLE XII Exemplary Plate X₄ X₅ X₆ 8 about 36.5 mm about 28.0mm about 14.0 mm 9 about 40.5 mm about 32.0 mm about 16.0 mm 10 about44.5 mm about 36.0 mm about 18.0 mm 11 about 48.5 mm about 40.0 mm about20.0 mm 12 about 52.5 mm about 44.0 mm about 22.0 mm 13 about 56.5 mmabout 48.0 mm about 24.0 mm 14 about 60.5 mm about 52.0 mm about 26.0 mm

Thus, kits of plates may be provided including two or more plates, suchas plates selected from exemplary plates 8-14 as listed in Table XII.For example, plates may be provided with lengths X₄ that differ by about4 mm each, lengths X₅ that differ by about 4 mm each, and lengths X₆that differ by about 2 mm each.

In addition, as shown for example in FIG. 25A, each of the exemplary“three-level” plates according to the present invention may be providedwith maximum lengths X₇, lengths X₈ between the centers of cephalad andcaudal holes, and lengths X₉ between the centers of adjacent holes, asfollows: TABLE XIII Exemplary Plate X₇ X₈ X₉ 15 about 53.5 mm about 45.0mm about 15.0 mm 16 about 56.5 mm about 48.0 mm about 16.0 mm 17 about59.5 mm about 51.0 mm about 17.0 mm 18 about 62.5 mm about 54.0 mm about18.0 mm 19 about 65.5 mm about 57.0 mm about 19.0 mm 20 about 68.5 mmabout 60.0 mm about 20.0 mm 21 about 71.5 mm about 63.0 mm about 21.0 mm22 about 74.5 mm about 66.0 mm about 22.0 mm 23 about 77.5 mm about 69.0mm about 23.0 mm

Thus, kits of plates may be provided including two or more plates, suchas plates selected from exemplary plates 15-23 as listed in Table XIII.For example, plates may be provided with lengths X₄ that differ by about3 mm each, lengths X₅ that differ by about 3 mm each, and lengths X₆that differ by about 1 mm each.

Also, as shown in FIG. 26A, each of the exemplary “four-level” platesaccording to the present invention may be provided with maximum lengthsX₁₀, lengths X₁₁ between the centers of cephalad and caudal holes, andlengths X₁₂ between the centers of holes of a two-level construct, asfollows: TABLE XIV Exemplary Plate X₁₀ X₁₁ X₁₂ 24 about 68.5 mm about60.0 mm about 30.0 mm 25 about 72.5 mm about 64.0 mm about 32.0 mm 26about 76.5 mm about 68.0 mm about 34.0 mm 27 about 80.5 mm about 72.0 mmabout 36.0 mm 28 about 84.5 mm about 76.0 mm about 38.0 mm 29 about 88.5mm about 80.0 mm about 40.0 mm 30 about 92.5 mm about 84.0 mm about 42.0mm 31 about 96.5 mm about 88.0 mm about 44.0 mm 32 about 100.5 mm about92.0 mm about 46.0 mm 33 about 104.5 mm about 96.0 mm about 48.0 mm 34about 108.5 mm about 100.0 mm about 50.0 mm

Thus, kits of plates may be provided including two or more plates, suchas plates selected from exemplary plates 24-34 as listed in Table XIV.For example, plates may be provided with lengths X₁ that differ by about4 mm each, lengths X₂ that differ by about 4 mm each, and lengths X₃that differ by about 2 mm each.

In addition, kits with one or more plates selected from the group of“one-level”, “two-level”, “three-level”, and “four-level” plates may beprovided. Thus, a kit may optionally include one or more plates selectedfrom exemplary plates 1-7, one or more plates selected from exemplaryplates 8-14, one or more plates selected from exemplary plates 15-23,and one or more plates selected from exemplary plates 24-34, as listedin Tables XI to XIV.

In another exemplary embodiment of the present invention, an exemplaryplate 1600 is provided for use without captive clips. As shown in FIGS.38A to 38G, plate 1600 includes fastener holes 1602 provided thereinwith opposed gussets 1604, 1606. Gussets 1604, 1606 serve a similarfunction as the clips disclosed with respect to other fixation systemsdescribed above, in that when a fastener is disposed in a hole 1602, thegussets are accommodated in groove 1610 in the head of the fastener.Thus, unlike the clips disclosed herein which may be plasticallydeformed to accommodate passage of the fastener and lock the fastener inplace, gussets 1604, 1606 are immobile and thus do not deform. As shownin FIG. 38E, fastener 1608 may be provided with slits 1612 extendingfrom an upper portion of head 1614 toward shaft 1616. Slits 1612 permitthe head of fastener 1608 to expand and contract in a spring-likefashion, thus accommodating the decreased width of hole 1602 at gussets1604, 1606 when fastener 1608 is installed or removed from hole 1602. Inparticular, fastener 1608 may be captured in hole 1602 as permitted whenthe head of fastener 1608 expands such that gussets 1604, 1606 arereceived in groove 1610. Optionally, head 1614 may be internallythreaded. Advantageously, the need for a clip is eliminated in platedesigns using gussets, allowing a one-piece plate construct requiringless manufacturing parts and easing assembly. Shown in FIG. 38G, thefastener 1608 is in a dynamic position; the fastener head 1614 is shownin the contracted state that occurs during insertion or removal offastener 1608 from plate 1600. Advantageously, gussets 1604, 1606provide the same function as described previously with respect tocaptive clip 680 shown in FIG. 30C.

Another embodiment of a fixation system is illustrated in FIGS. 39A-42B.FIG. 39A shows a perspective view of a plate 750 which may have aplurality of fixation holes 751, and drill guide keys 752. Fixationholes 751 may receive bone fasteners, such as fixed-angle bone screws760 or variable-angle bone screws 770. Drill guide keys 752 may receivea portion of a drill guide (not shown), which may assist in aligning adrill barrel for more accurate use of a drill or other instrumentthrough a fixation hole 751. Drill guide keys 752 may be a variety ofshapes and sizes (including circular, triangular, elliptical, polygonal,and rectangular), and may be placed a varying or uniform distances awayfrom fixation hole 751. Drill guide keys 752 may also be utilized foraligning a guide over more than one fixation hole 751. Plate 750 mayalso have windows 753, which may assist in visualization of a bodytissue or bone graft below plate 750. Windows 753 may be configured,sized, and/or located such that when plate 750 is attached to a patient,at least one window 753 overlies an intervertebral disc space. Windows753 may be a variety of shapes and sizes, including but not limited todiamond-shaped, circular, triangular, elliptical, polygonal, andrectangular. Clip 755 may also be disposed within a groove 759 (see FIG.39D) fixation hole 751, and may engage a bone screw 760, 770 to preventscrew back-out. Clip 755 (described in more detail in below and inrelation to FIG. 40), may have any or all of the characteristics orspecifications of the clips described above. A top view of plate 750 isseen in FIG. 39B.

FIG. 39C shows a side view of plate 750 having a plurality ofindentations 757, which may be beneficial for pre-operative bending ofthe plate 750 to better fit a patient's anatomy. Indentations 757 may bea variety of shapes, and may be placed at various locations along theplate 750. Although seen along the lower surface of plate 750 in FIG.39C, indentations 757 may be placed on a variety of plate 750 surfaces,including the top surface and/or side surfaces, and/or along theperimeter of the plate 750.

FIG. 39D shows a cross-sectional view of the plate of FIG. 39C takenthrough the line X39-X39. As discussed above, clip 755 may be disposedwithin a groove 759 in a fixation hole 751. Fixation hole 751 may havetapered surfaces 758 a, 758 b within the inner surface of the fixationhole 751.

FIG. 40 shows a top view of another embodiment of a clip 755 for usewith a variety of plates described herein. Clip 755 may be symmetrical.Clip 755 may have a notch 755 a, and two flanges 755 b, 755 c. Flanges755 b, 755 c may have ends 755 f, 755 g. Flanges 755 b, 755 c may alsohave recessed portion 755 d, 755 e, and a thickened portion 755 h, 755i. At least a portion of clip 755, particularly the thickened portions755 h, 755 i, may engage a bone fastener (such as screws 760, 770)inserted into a fixation hole 751.

Variable-angle screw 770 and fixed-angle screw 760 are shown in moredetail in FIGS. 41A-42B. A side view of an embodiment of avariable-angle screw 770 is seen in FIG. 41A. Variable-angle screw 770may have a head portion 772, a shaft 773 with at least one thread 771,and a tip 778. Head portion 772 may have a trailing ledge 774, anannular ledge 775, and a convex surface 776 extending from the annularledge 775 to the shaft 773. Head portion 772 may also have an engagingportion 777 for receiving an instrument, such as a screwdriver. Theengagement of screw 770 with a fixation hole 751 is discussed below inrelation to FIG. 42A.

A side view of an embodiment of a fixed-angle screw 760 is seen in FIG.41B. Fixed-angle screw 760 may have a head portion 762, a shaft 763 withat least one thread 761, and a tip 768. Head portion 762 may also have atrailing ledge 764, an annular ledge 765, and an annular groove disposedbetween the annular ledge 765 and the shaft 762. Head portion 762 mayalso have an engaging portion 767 for receiving an instrument, such as ascrewdriver. The engagement of screw 760 with a fixation hole 751 isdiscussed below in relation to FIG. 42B.

FIG. 42A shows a variable-angle screw 770 inserted into a fixation hole751 of a plate 750, with a portion of the plate 750 removed for clarity.Clip 755 can be clearly seen disposed in groove 759. When variable-anglescrew 770 is fully inserted into fixation hole 751, clip 755 may bedisposed between trailing ledge 774 and annular ledge 775. Preferably,annular ledge 775 is sized such that after it passes below the clip 755,it may not back out of the fixation hole 751, thereby preventing overallscrew 770 back-out. Convex surface 776 of screw 770 may contact taperedsurface 758 a of fixation hole 751. Tapered surface 758 a may beconcave, and thus may provide a close fit with convex surface 776.Variable-angle screw 770 may assume a variety of angulations withinfixation hole 751, consistent with the discussion above.

FIG. 42B shows a fixed-angle screw 760 inserted into a fixation hole751, with a portion of the plate 750 removed for clarity. Clip 755,along with notch 755 a and end 755 f, can be clearly seen disposed ingroove 759. When fixed-angle screw 760 is fully inserted into fixationhole 751, clip 755 may be disposed between trailing ledge 764 andannular ledge 765. Preferably, annular ledge 765 is sized such thatafter it passes below the clip 755, it may not back out of the fixationhole 751, thereby preventing overall screw 760 back-out. Fixed-anglescrew 760 may has a predetermined angulation within fixation hole 751,consistent with the discussion above.

FIGS. 43A-44D show embodiments of a drill guide for use, inter alia,with the system of FIG. 39A. FIG. 43A shows a side view of avariable-angle drill guide 850, which may have a barrel 851, tip portion852, handle 853, and a connecting portion 854 for connecting the handle853 to the barrel 851. A cross-sectional view of barrel 851 and tipportion 852 is seen in FIG. 43B. Barrel 851 may have a bore 855 whichextends through the tip portion 852 as well. Bore 855, may bevariably-sized along the length of the barrel 851 and/or tip portion852. Preferably, bore 855 should be sized to receive a drill bit (notshown) or other instrument. Tip portion 852 is shown in more detail inFIG. 43C, in an enlarged cross-sectional view. Tip portion 852 may havea leading end 856. Tip portion 852 may also have an engaging portion 857for engaging a portion of a fixation hole 851. Engaging portion 857 maybe shaped and sized such that variable-angle drill guide 850 may assumea variety of angulations in relation to a fixation hole 751, therebyenabling a hole to be drilled at a variety of angulations into a bonesegment beneath fixation hole 751.

FIG. 44A shows a side view of a fixed-angle drill guide 870, which mayhave a barrel 871, tip portion 872, handle 873, and a connecting portion874 for connecting the handle 873 to the barrel 871. A cross-sectionalview of barrel 871 and tip portion 872 is seen in FIG. 44B. Barrel 871may have a bore 875 which extends through the tip portion 872 as well.Bore 875, may be variably-sized along the length of the barrel 871and/or tip portion 872. Preferably, bore 875 should be sized to receivea drill bit (not shown) or other instrument. Tip portion 872 is shown inmore detail in FIG. 44C, in an enlarged cross-sectional view. Tipportion 872 may have a leading end 876. Tip portion 872 may also have anengaging portion 877 for engaging a portion of a fixation hole 871.Engaging portion 877 may have a protruding ledge 878, which may bedisposed immediately adjacent a clip 755 in a fixation hole 751, as seenin FIG. 44D. Engaging portion 877 may be shaped and sized such thatfixed-angle drill guide 870 may only assume a predetermined angulationwith a fixation hole 751. This is achieved with exemplary drill guide870 by having a protruding ledge 878 that extends to the tapered surface758 a of fixation hole 751. The result may therefore be that tip portion872 of fixed-angle drill guide 870 may only fit in a fixation hole 751with a single orientation, as compared to variable-angle drill guide850, which may assume a variety of angulations.

Drill guides 850, 870 may further have more than one barrel. Handle 853,873 may be removably associated with a barrel 851, 871. Preferablyhandle 853, 873 is sized to accommodate a user's hand.

While various descriptions of the present invention are described above,it should be understood that the various features can be used singly orin any combination thereof. Therefore, this invention is not to belimited to only the specifically preferred embodiments depicted herein.

Further, it should be understood that variations and modificationswithin the spirit and scope of the invention may occur to those skilledin the art to which the invention pertains. For example, each of theplates described herein preferably are provided with curvature toconform to the spinal anatomy. In alternate embodiments, however, theplates instead may be provided without pre-lordosis. In addition, eachof the plates described herein instead may be provided in embodimentsthat only include two fixation holes. Accordingly, all expedientmodifications readily attainable by one versed in the art from thedisclosure set forth herein that are within the scope and spirit of thepresent invention are to be included as further embodiments of thepresent invention. The scope of the present invention is accordinglydefined as set forth in the appended claims.

1. A bone plate comprising: a first pair of fixation holes configured tolie adjacent a first vertebrae; a second pair of fixation holesconfigured to lie adjacent a second vertebrae; and a first aperturedisposed between the first pair of fixation holes and the second pair offixation holes, and configured to overlie the disc space between thefirst vertebrae and the second vertebrae; wherein the first apertureallows for increased visualization of the disc space when the bone plateis adjacent the first and second vertebrae.
 2. The bone plate of claim1, further comprising a first drill guide key for engaging a drillguide.
 3. The bone plate of claim 2, wherein the first drill guide keyis adjacent a fixation hole.
 4. The bone plate of claim 1, furthercomprising a third pair of fixation holes and a second aperture disposedbetween the second pair of fixation holes and the third pair of fixationholes.
 5. The bone plate of claim 1, wherein the first aperture issubstantially elongate.
 6. The bone plate of claim 1, further comprisinga longitudinal axis, and wherein the first aperture is substantiallyaligned with the longitudinal axis.
 7. A bone plate comprising: a topsurface; a bottom surface; a plurality of fixation holes extending fromthe top surface to the bottom surface, and for receiving a bone screw;and at least a first indentation disposed along the bottom surface, andconfigured to promote pre-operative bending of the bone plate.
 8. Thebone plate of claim 7, wherein the first indentation is substantiallycurved.
 9. The bone plate of claim 7, wherein the first indentation isdisposed along the outer perimeter of the bone plate.
 10. A bone platecomprising: first and second fixation holes for receiving a bone screw;first and second drill guide keys for engaging a drill guide; and afirst aperture separate and distinct from the fixation holes and thedrill guide keys; wherein the first drill guide key is disposed adjacentthe first fixation hole, and that second drill guide key is disposedadjacent the second fixation hole.
 11. The bone plate of claim 10,wherein the first aperture is substantially larger than both the firstand second drill guide keys.
 12. A fixation system comprising: a boneplate having at least a first fixation hole for receiving a bone screw,and a groove in communication with the first fixation hole; a resilientclip disposed the groove; and a drill guide having a tip portionconfigured to engage the resilient clip when the tip portion is insertedinto the first fixation hole.
 13. The fixation system of claim 12,wherein the resilient clip is configured to engage a bone screw.
 14. Thefixation system of claim 12, wherein the tip portion of the drill guidemay engage the first fixation hole at a variety of orientations.
 15. Thefixation system of claim 12, wherein the tip portion of the drill guidemay engage the first fixation hole at only one orientation.
 16. Thefixation system of claim 12, wherein the drill guide further comprises abarrel for receiving an instrument.
 17. The fixation system of claim 12,wherein the tip portion of the drill guide is shaped such that once thetip portion engages the resilient clip, the tip portion is preventedfrom backing out of the first fixation hole without user intervention.